RF power amplifiers are used in wireless communications such as in base stations and mobile phones in a cellular radio network, or as a further example, in wireless access points and wireless computing devices in a WiFi network. Conventional analog RF transmitter architectures are difficult to design to achieve good linearity and power efficiency simultaneously. Good linearity results in high power consumption, because the amplifier in the power amplifier architecture operates as a Class A or Class AB amplifier, resulting in poor power efficiency in a power back-off mode. Analog power amplifiers have other disadvantages including a requirement of a large surface area, difficulty of integration on a chip, and a requirement of matching networks between an RF integrated circuit (RFIC) and the power amplifier (PA).
Digital modulated RF power amplifiers have the potential to achieve better power efficiency and linearity than analog power amplifiers, since pre-distortion can be performed more accurately and with lower energy consumption. This is helpful in mobile devices such as mobile phones that are powered by batteries with limited life. Digital modulated RF power amplifiers may generally be classified into two categories: quadrature modulated PAs and polar modulated PAs. Quadrature modulated PAs have two balanced amplitude modulation (AM) paths and polar modulated PAs have one AM path and one phase modulation (PM) path.
A polar modulated PA may operate at higher efficiency than a quadrature modulated PA. However, countering band expansion in a polar modulated PA is difficult, especially for wideband applications such as the Long Term Evolution (LTE) systems, where baseband signal bandwidth is as wide as 20 Megahertz (MHz), and the shortest duplex distance for a frequency division duplex band is 80 MHz. Due to signal bandwidth expansion in a polar modulated PA, adjacent channel interference and error vector magnitude (EVM) is high. Another disadvantage of polar modulated PAs is mismatch between the AM path and the PM path. These two paths are physically and electrically quite different, yet the maximum allowable delay mismatch between them should be less than a nanosecond, which is very difficult to achieve.
A quadrature modulated PA, also called an IQ modulated PA, has lower power efficiency than a polar modulated PA, has less bandwidth expansion, and has symmetric channels. Thus, the quadrature modulated PA is easier to implement and is more suitable for wideband applications such as LTE, but is not as efficient as other technologies.
Various references are directed to design of features of power amplifiers and power supplies. In “Optimum Bias Calculation for Parallel Hybrid Switching Linear Regulators”, Stauth, et. al, IEEE Transactions on Power Electronics, vol. 22, no. 5, September 2007, pages 1978-1985, bias constraints for switching and linear voltage regulators are analyzed and ideal expressions for the optimum current contribution of the switching regulator are derived. The article entitled “Efficiently Amplified,” Kim, et. al, IEEE Microwave Magazine, vol. 11, no. 5, August 2010, pages 87-100, generally discusses supply modulated power amplifiers. U.S. Patent Application Publication No. 2010/0208848 describes a quadrature modulator for producing I and Q channel signals. “A New Power Management IC Architecture for Envelope Tracking Power Amplifier,” Choi, et. al., IEEE Transactions on Microwave Theory and Techniques, vol. 59, no. 7, July 2011, pages 1796-1802, describes a power supply modulator for use with a power amplifier. U.S. Patent Application Publication No. 2005/0064830 describes a DC to DC converter having a switch mode part and a linear mode part. U.S. Pat. No. 7,139,534 describes a modulated signal generator having a phase component line and an amplitude component line. U.S. Patent Application Publication No. 2009/0267581 describes a switched mode power supply and a linear mode power supply.
Two common sources of reduced power efficiency in both polar modulated and quadrature modulated PAs are (1) power back off mode and (2) large peak to average power ratio (PAPR) in multicarrier based modulation methods such as orthogonal frequency division multiplexing (OFDM) used in LTE. As a consequence of these two factors, a PA operates at a very low average power level yet has a high PAPR, leading to very low power efficiency.